The present invention relates to a low noise microwave amplifier wherein a rectangular waveguide is used in its signal input part.
In a conventional microwave amplifier such as a LNA (Low Noise Amplifier) for exclusive use in a television receiver in 4 GHz-band, a down-converter for receiving from DBS (Direct Broadcasting Satellite) in 12 GHz-band, or the like, a rectangular waveguide is used in its signal input part. In such an amplifier, a waveguide microwave mode is converted to a coaxial mode (microstrip mode) by means of a waveguide-to-coaxial converter portion shown in FIGS. 4(a) and 4(b), and the converted microwave is then amplified by the use of a silicon bipolar transistor or FET (Field Effect Transistor). FIG. 4(c) shows a block diagram of the first half portion of an amplifying portion by means of the microwave stripline. In FIGS. 4(a) and 4(b), numeral 1 is a waveguide standardized in JIS (Japanese Industrial Standard) having a flange 1a to be connected to a corresponding waveguide. Numeral 2 is an amplifying portion of which input part is composed of a microstrip line equivalent to a coaxial cable. Numeral 3 is a probe for detecting a microwave (TE wave) in the waveguide 1 and converting it into a TEM wave to transmit the wave into the above microstrip line.
In such an low noise microwave amplifier as described above, an oscillation easily takes place when the gain of the amplifier is set at high level. As main causes of an oscillation in the microwave amplifier, in general, there can be given that the gain of the amplifier is high; the impedance of the previous stage with respect to an input part of the FET is in an unstable region; and there exists an input such as noise power which might become a cause of oscillation, and the like.
FIG. 5 shows a relationship between the frequency and the gain in a low noise microwave amplifier of 4 GH.sub.z -band having the above-mentioned construction. The necessary amplifying bandwidth of an output of the amplifier is from f.sub.L (=3.7 GH.sub.z) to f.sub.H (=4.2 GH.sub.z) in FIG. 5. It is preferable that the amplification occurs within the above range (from f.sub.L to f.sub.H) In fact, however, there occurs the amplification below the frequency f.sub.L and/or over the frequency f.sub.H. In this example, as shown in FIG. 5, there exists a noise source, at a frequency of 3.4 GH.sub.z, of which gain is larger than the desired signal. The noise is considered to generate an oscillation. In order to examine in detail the above causal relation, the relationship between the impedance and the frequency at the first half portion of the amplifier, which is observed from the input part of FET, is investigated. The result is shown in FIG. 6 in the form of a schematic Smith Chart. The impedance generally depends on the frequency because impedance consists of reactance. As is clear from FIG. 6, the frequency of around 3.4 GH.sub.z is just in the unstable operation region shown as a hatched portion A in FIG. 6. It is concluded that the above oscillation is due to this fact. Though it is possible to shift the impedance to the stable region by adjusting the stripline matching lines .theta..sub.1, .theta..sub.2, and .theta..sub.3, the impedance of other frequency also changes and shifts to the above-mentioned unstable region. These matching lines are generally adjusted so as to give the best NF (Noise Figure) at so-called amplifying bandwidth (from f.sub.L to f.sub.H). Thus, hitherto, in order to avoid the oscillation, an isolator is provided between the amplifying portion 2 and the waveguide 1 to prevent the oscillation. According to the above isolator, the impedance at the input side of the amplifier is fixed to reduce the effect of fluctuation of the impedance at the input side of the isolator. However, in a method as mentioned above, there are generated problems that the constitution becomes complicated, it costs much to provide the isolator and a reduction of noise figure corresponding to an insertion loss of the isolator is brought about.
Another method is also employable wherein a certain filter is inserted at the input side of the microwave amplifier to eliminate a frequency component below the frequency f.sub.L which is the lower limit of the amplifying bandwidth. In such a case, a reduction of noise figure corresponding to an insertion loss of the filter takes place. In order to prevent the above reduction, the filter is inevitably provided at the following part of the amplifier instead of the input side of the amplifier. In that case, however, the effect of the filter is greatly lessened.
An object of the invention is to provide a low noise microwave amplifier capable of greatly reducing a possibility of oscillation of a microwave amplifier without using an isolator, filter, or the like, even when a device of every impedance is connected to the input part of the converter.
This and other objects of the invention will become apparent from the description hereinafter.